1. Field of the Invention
The invention relates to a process for connecting a molybdenum foil to a molybdenum lead portion. The invention furthermore relates to a production process for producing a hermetically enclosed part of a lamp using the resultant connected foil and lead part.
2. Description of Related Art
Conventionally, quartz glass is often used as lamp material; however, it has a coefficient of expansion significantly different from that of the tungsten or molybdenum of which the lead pin is normally made. Therefore, to form a hermetically enclosed part, a direct hermetic enclosure of the quartz glass on the lead pin is not used, and instead, a welding of the lead pin to a molybdenum foil is performed. In this way, an electrical connection can be maintained inside and outside of the lamp.
FIG. 5 illustrates a hermetically enclosed part of a conventional filament lamp. In the representation, numeral 1 references a bulb made of quartz glass; on an end, a hermetically enclosed part 11 is formed in which molybdenum foil 4 is placed. An inner lead 3 is connected to filament 2 and the inner lead 3 and an outer lead 5 both resistance welded to molybdenum foil 4 for providing an electrical connection enabling an external source of power to be applied to the filament. Tungsten or molybdenum is used for these leads.
The molybdenum foil 4 is resistance welded to an end 31 of inner lead 3 and an end 51 of the outer lead 5 with either platinum of a relatively low melting point or molybdenum with platinum coating (platinum clad molybdenum) being disposed therebetween as a binder 7. In this binder 7, for example, the platinum clad molybdenum is formed such that a molybdenum foil 71 is coated with a platinum film 72 as shown in FIG. 6. Molybdenum foil 71 has, for example, a thickness of 28 microns, and the platinum film 72 has a thickness of 1 micron.
When, in this case, the leads and the molybdenum foil are temporarily welded directly to one another, as a result of their temperature increases, oxidation and nitration occur, and mechanical strength decreases even if welding is achieved. In particular, in the molybdenum foil, due to a small tensile force, holes can be formed since its temperature rises slightly due to its small thickness.
The molybdenum and the tungsten which comprise the leads are formed as pins of sintered metals and consist of fine crystal grains which adhere to one another when they are exposed to a high temperature. This phenomenon is usually called recrystallization, by which the lead pins consisting of the fine crystal grains change into lead pins consisting of large crystal grains. According to this change, the lead pins are inherently fragile, and mechanical strength, likewise, decreases.
For these reasons, instead of direct welding, a binder is used, whereby the platinum with a low melting point is melting first as the binder, and thus, the molybdenum foil and the lead pins are able to be joined to one another. This means that, when using the binder, less electrical energy can be used in welding. Therefore, the temperature rise of the molybdenum foil and the lead pins can be reduced, and thus, nitration and oxidation of the above-described molybdenum foil and above-described lead pins is prevented along with an associated reduction of mechanical strength due to recrystallization.
In addition, the temperature rise of the molybdenum foil and the lead pins is reduced by removing the electrical energy of welding through the binder, such as a platinum foil or the like. In this way, adhesion of the molybdenum foil and the lead pins to the welding electrode rods during welding is prevented, and the advantage is gained that welding can be done in a relatively simple manner.
However, in this case, it is considered disadvantageous that platinum is an expensive precious metal and thus raises costs. Furthermore, there is the disadvantage that, as the result of different coefficients of expansion of the platinum and quartz glass which comprises the bulb, cracking occurs in hermetically enclosed part 11.
Proceeding from the above-described circumstances, different measures are known, for example, from the published Japanese utility model SHO 53-13251 and Japanese patent SHO 63-40354, in which the molybdenum foil and the lead portions are welded directly to one another without using a binder, such as platinum, platinum clad molybdenum or the like. By means of the measures disclosed therein, an attempt is made to reduce the contact area of the molybdenum foil with the lead portions and to increase welded strength by intensification of a welding current by changing the shape of the lead portion.
However, in these measures, it is considered disadvantageous that the mechanical strength is less as compared to using a binder, such as platinum, platinum clad molybdenum or the like, and that the phenomenon of nitration, oxidation and recrystallization occurs in the molybdenum foil and lead pins as the result of increasing the welding current for purposes of increasing the strength.
In particular, in the case of manufacturing a filament lamp with bilateral hermetic seals, the two ends of a filament assembly which formed of an inner lead, a molybdenum foil for purposes of hermetic enclosure, and an outer lead, are hermetically sealed by exerting a tensile force on the ends of the filament assembly during heating of the quartz bulb. It is, therefore, necessary to weld the inner lead and the outer lead to the molybdenum foil for purposes of hermetic enclosure with high mechanical strength.